In cases of skeletal injuries such as broken bones and cervical spinal injuries, it is usually necessary to fix or stabilize the position of the bones or the head of a patient while the injury heals or undergoes treatment. Such fixation may require the attachment of an external fixation frame to the long bones or a traction device or halo orthosis to the skull of a patient. A commonly used device for attaching a frame to a bone structure is the fixation pin. The halo orthosis or halo traction device provides cervical spine immobilization by penetration of the outer table of the skull with fixation pins passed through a ring or "halo" surrounding the skull and attached to a rigid body vest or cast or to a traction mechanism. As used herein, "halo orthosis" refers to the entire support apparatus and "halo" refers to the ring or frame which fully or partially surrounds the skull. The external fixation frame provides long bone immobilization by penetrating each segment of the broken bone with fixation pins attached to a rigid frame.
Fixation pins are well known in the art. Typically, halo type fixation pins are screw-like devices having a threaded cylindrical body, a conical tip at the front end for penetrating at least the outer surface of a bone, and a head or depression at the rear end for coupling to a means for rotating the pin. Long bone fixation pins are typically threaded and may be self-tapping.
Many types of fixation pins have been devised and described in patents and publications. U.S. Pat. No. 2,966,383, to Carl H. Boetcker and William A. Shafer, 1960, discloses a surgical headrest utilizing a pin having a spring-loaded conical tip with a surrounding flange.
U.S. Pat. No. 4,978,350, to Marcel H. Wagenknecht, 1990, discloses a fixation pin having flutes similar to a twist drill cut into the threaded portion of the pin.
U.S. Pat. No. 5,522,817, to Thomas W. Sander and Jeffery S. White, 1996, discloses a surgical bone pin having a bone penetrating element and a bioabsorbable shaft of uniform diameter.
An article "Structural Behavior of the Halo Orthosis Pin-Bone Interface: Biomechanical Evaluation of Standard and Newly Designed Stainless Steel Halo Fixation Pins" by Steven R. Garfin et al. from Spine, Vol. 11, No. 10 of 1986, pages 977-981 contains FIGS. 1 and 2 on page 978 showing various pin designs. These two figures are reproduced herein as FIGS. 1A-1G and FIGS. 2A-2B, respectively. The alphabetic portions of the figure numbers for FIGS. 1A-1G and FIGS. 2A-2B herein correspond to the alphabetic reference characters in FIGS. 1 and 2 of the article.
FIGS. 1A-1G are schematic diagrams of one standard (control) pin design and six experimental pin designs: standard tip (FIG. 1A), conical tip (FIG. 1B), cylindrical tip with sharp cutting edge (FIG. 1C), cylindrical tip with sharp cutting teeth on edge (FIG. 1D), narrow bullet tip (FIG. 1E), wide bullet tip (FIG. 1F), and bullet tip and raised cylindrical shoulder with cutting edge FIG. (1G). The authors of the article stated that "All designs but one proved unsatisfactory because of inadequate (FIG. 1: C, D, E) or excessive (FIG. 1: B, G) pin penetration into the calvarium. The remaining pin design (FIG. 1: F, and FIG. 2) used a stainless steel cylindrical tip 1.5 mm in diameter with a sharp, slightly tapered point machined on the end. The distance from the tip of the pin to the shoulder measured 2.5 mm. The choice of this distance was based on previous measurements of the mean thickness of the outer cortex of the adult calvarium. This pin design enables the cylindrical column of the pin to penetrate the outer table of the adult calvarium, but further penetration is limited by the shoulder of the pin. As a result, adequate pin penetration is achieved but excessive penetration with its serious complications is avoided."
FIGS. 2A and 2B are schematic diagrams showing in greater detail the experimental wide bullet tip design of FIG. 1F and the standard tip design of FIG. 1A, respectively. The control pin having the standard tip design was described in the article as a commercially available stainless steel halo fixation pin (Jerome Medical, Mt. Laurel, N.J.).
With the potential for catastrophic complications associated with inadequate immobilization of unstable cervical spine injuries, it is important to maintain a stable connection between a halo and the skull of the patient. However, loosening of the fixation pins at the pin-bone interface is the most common complication associated with use of the halo orthosis. Pin loosening rates as high as 60% have been reported. Conclusions have typically been that loosening is reduced if the insertion torque and subsequent pin tip penetration are increased resulting in a larger axial pin fixation force into the skull surface.
Conventional conical or tapered pin tips require high axial forces to insert and hold the pins in place without loosening. For example, the aforementioned article states that a torque of 8 in-lb (0.9 N-m) applied to halo pins has been found to be safe and effective in lowering the incidence of pin loosening and infection. Eight in-lb of torque applied to a fixation pin can produce axial forces in the range of 100 to 200 pounds (450 to 900 N) depending on the material of the pin, lubrication, etc. Such high forces can be quite uncomfortable for the patient. For example, headaches are a well documented side effect of halo fixators.
It is therefore an object of this invention to provide a fixation pin which can be inserted and held in place with a small axial force and with little damage to the surrounding bone, and which can withstand repetitive transverse loading without loosening. Another object of the invention is to provide a halo orthosis which affords more comfort and better peripheral vision for the patient, greater flexibility in pin placement, and better access to the spine for treatment. A further object of the invention is to provide a method of attaching an external fixation frame to a long bone which reduces the risk of damage to tissue surrounding the bone.